Light-emitting diodes (LEDs) may be used for the light sources of liquid crystal display devices. Area control can be applied to LED light sources. Thus, it is possible to display an image with high quality such as high brightness and wide color gamut. In area control, the lighting value of the LED of each lighting area is calculated based at least in part on pixel data corresponding to the lighting area for each frame. A current and a lighting time are controlled based at least in part on the lighting value. As the lighting time is controlled by a pulse width modulation (PWM) of the lighting time, the lighting time control may be also called a PWM control. When a white LED as a backlight is driven by combining the current control and the PWM control of the lighting time, the emission color of the LED can be controlled by the current, and the brightness can be stably controlled by the PWM.
The properties of LEDs are uneven. Thus, the gradation control of the lighting value is unstable by only the current control. The stable change in brightness and improvement of dynamic characteristics are realized by the PWM-controlling the lighting time in a state where a current is kept constant.
It is necessary to considerably reduce the lighting time (pulse width) to decrease the brightness. However, if the pulse width is narrow, the PWM control of the lighting time may be unstable because of the relationship between the pulse width and the timing of supply of a constant current to the LED. Regarding dark portions in which the brightness is difficult to control by the PWM control of the lighting time, a method for stabilizing the gradation of the lighting value by the current control is proposed. In this method, the current control is performed with a fixed pulse width in dark portions. In the other portions (bright portions), the PWM control of the lighting time is performed with a fixed current (the maximum value in the current control).
For the LED light source, LEDs of three colors of red, green and blue are used. Further, a white LED prepared by combining fluorescent elements emitting green light and red light with blue light having a short wavelength as an excitation light source is used. A white LED may use a phosphorescent element which emits phosphorescence as well as fluorescence to realize a wide gamut. Fluorescence disappears at once (in approximately one billionth of a second to one hundred-thousandth of a second).
However, phosphorescence disappears in approximately one thousandth of a second to several seconds. Thus, when an object moves at high speed on a screen, phosphorescence may remain in eyes after the movement of the object. Thus, the image quality may be degraded. When the white LED which emits phosphorescence is driven by performing the current control in dark portions and performing the PWM control of the lighting time in bright portions, an afterglow may occur in accordance with the pulse width of the PWM. When the PWM value (the duty ratio of the pulse width) is 100%, no problem occurs. However, it is known that the degradation of the display quality by the afterglow of phosphorescence is noticeable when the pulse width is decreased to a width.